System for measuring the speed of rotation of a synchro by means of a sampling technique

ABSTRACT

By combining three single-phase output voltages of a synchro, signals are obtained which are proportional to the sine and the cosine of the angle of rotation of the synchro shaft. Said signals are sampled at predetermined instants by switching means controlled by a clock voltage, and are applied to computing means which form a signal representative of the speed of rotation of the shaft of the synchro and of a change in the altitude at which an aircraft flies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system for generating a signal which isrepresentative of the speed of rotation of a synchro while using a firstdigital signal which is proportional to the sine of the angle ofrotation of the shaft of the synchro, and a second digital signal whichis proportional to the cosine of said angle.

2. Description of the Prior Art

The signals which correspond to the speed of rotation of the synchro areused in many fields of application, particularly for aircraft which arenot provided with a device for indicating their vertical speed. In thiscase signals are utilized originating from a synchro (driven by abarometric altimeter) the speed of rotation of which is proportional tothe vertical speed H_(B) of the aircraft.

In a known system an analog method is used for deriving the outputvoltage for generating the speed signal, which causes problems asregards with respect to the sensitivity, the phase distortion, theharmonic distortion of the output noise and the excitation voltage ofthe synchro.

To avoid the difficulties encountered when generating a signal which isequal to the derivative of an analog signal, it is possible to use thedigital sine and cosine signals for calculating the tangent of the angleof rotation of the shaft of the synchro and the derivative of thetangent. As the tangential function is only linear for small angles itis impossible to approach this derivative by means of a first term of aseries expansion but several terms must be used, which requires acomplicated circuit.

SUMMARY OF THE INVENTION

The invention is characterized in that the system according to theinvention comprises means for a first sampling of said sine signal,means for a first sampling of said cosine signal, means for a secondsampling of said sine signal after a predetermined time after the firstsampling of the sine signal has elapsed, means for a second sampling ofsaid cosine signal after a predetermined time after the first samplingof the cosine signal has elapsed, means for multiplying said secondsampled sine signal by said sampled first cosine signal for forming afirst product, means for multiplying said sampled first sine signal bysaid sampled second cosine signal for forming a second product, meansfor squaring said sine and cosine signals, means for adding said squaredsine and cosine signals for forming a first sum, means for dividing saidfirst product by said first sum for forming a first quotient, means fordividing said second product by said first sum for forming a secondquotient, and, finally, means for subtracting said second quotient fromsaid first quotient for producing in that manner a signal which isrepresentative of the speed of rotation of the synchronous machine.

The advantages of the system according to the invention will beexplained with reference to a special embodiment of the system. Thebasic principle of the invention is as follows:

The variation Δφ of the angle of rotation of the shaft of the synchro isgiven by the formula: ##EQU1## where T denotes the first instant atwhich the generated sine and cosine signals are sampled for the firsttime, T+Te the second instant at which said sine and cosine signals aresampled for the second time, while Δφ denotes the change in the angle ofrotation of the shaft of the synchro between said first sampling instantand said second sampling instant.

The sine and cosine signals generated by the synchro are applied to atransformer whose transformation ratio is denoted by K, so that thevoltages at the terminals of the secondary windings S1 and S2 of saidtransformer have the following values:

    E.sub.s1 =K·sin φ·cos φωt,

    E.sub.s2 =K·cos φ·cos φωt.

At the sampling instants T and (T+Te) the voltage values of theenvelopes of the expressions K sin φ cos ωt and K cos φ cos ωt are givenby the formula:

    E.sub.s1 =K·sin φ,

    E.sub.s2 =K·cos φ.

The component cos ωt represents the instantaneous amplitude of themodulated voltage, that is to say of the excitation voltage of thesynchro.

sin Δφ expressed in the voltage at the secondary side of the transformeris as follows:

    sin (Δφ)=K.sup.2 {sin φ(T+Te) cos φ(T)-sin φ(T) cos φ(T+Te)}.

The values sin (Δφ) depends on the transformation ratio K which, in itsturn, depends on the excitation voltage of the synchro. To eliminatethis noise source use is made of the relation between thetrigonometrical functions:

    sin.sup.2 φ(t)+cos.sup.2 φ(t)=1, so K.sup.2 {sin.sup.2 φ(t)+cos.sup.2 φ(t)}=K.sup.2.

The generated sine and cosine signals are demodulated, squared and addedto obtain the expression:

    K.sup.2 [sin.sup.2 φ(t)+cos.sup.2 φ(t)],

which is thereafter used for dividing the value sin (Δφ): ##EQU2##

So the value sin (Δφ) only depends on the time interval Δt between thefirst and second sampling instants T and (T+Te). The shorter timeinterval Δt, the smaller is the measured angle Δφ, that is to say thevariations in the speed of rotation of the synchro can be distinguishedmore properly. Owing to the implementation of the system the first andsecond signal samples of the generated sine and cosine signals are nottaken simultaneously at the instants T and (T+Te).

Actually, the first sine signal is sampled at the instant T while thefirst cosine signal is sampled at the instant (T+T₁), while the sine andcosine signals are sampled for the second time at the instants (T+Te)and (T+Te+T₁), respectively. In practice this method results in anegligible error because the ratio between Te=(T+Te)-T and T₁ =(T+T₁)-Tis usually very large. Of course the instants Te and T₁ can be varied,if circumstances require so, by means of the central clock whichcontrols the synchronization between the various switching functions.This will be described in greater detail in the extensive descriptions.

For small values of Te, the value of sin (Δφ)≈Δφ, Δφ is proportional tothe speed of rotation of the synchro.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained with reference to a drawing inwhich:

FIG. 1 shows the circuit diagram of a system for generating a signalwhich is representative of the speed of rotation of the synchro, saidspeed of rotation corresponding to the speed variations of thebarometrical height H_(B) used in an aircraft,

FIG. 2 is a table showing all possible combinations of the signs of thesine and cosine signals,

FIG. 3 is a time diagram of the signals supplied by a central clock andwhich are indispensable for synchronizing the controls of the variousswitching functions.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 the synchro 3 supplies three monophase output voltages X, Yand Z. The reference or excitation voltage 11 is a 26 Volts, 400 Hz a.c.voltage, indicated by V in FIG. 1. A combining of the voltages at theoutput terminals X, Y and Z of the synchro will result in the generationof sine and cosine signals which are proportional to the angle ofrotation φ of the shaft of the synchro so that:

    V(XY)=sin φ cos ωt

    V(XZ+YZ)=cos φ cos ωt,

where cos ωt is the instantaneous amplitude of the excitation voltage.Said voltage combination is obtained by means of a transformer 12 inScott-arrangement and having a total transformation ratio K so that thefollowing voltages are produced at the terminals of the secondarywindings of the transformer:

    ES1=K sin φ cos ωt

    ES2=K cos φ cos ωt.

An additional function of said transformer consists in making thepotentials of the synchro independent of the potentials of thetachometrical system. The demodulators 13 and 14 form the envelopes ofthe sine and cosine signals, that is to say, the envelopes K sin φ and Kcos φ, respectively, from said voltages.

The sinusoidal output voltage of the demodulator 13 is stored in astorage circuit 17 after a first selection at the instant T by means ofa switch 16. In the same manner the cosinusoidal output voltage of thedemodulator 14 is sampled at the instant T via a switch 15 and stored ina storage circuit 18. Said switches 15 and 16 are closed only duringshort time intervals (some microseconds) which are defined by the pulsesderived from the clock pulse system 60 which will be describedhereafter.

It approximately holds that the first samples of the sine and cosinesignals are taken simultaneously at the instant T instead of at theinstants T and T+T₁ as explained above.

At the instant T+Te switches 20 and 22 are simultaneously closed so thatthe output voltage K sin φ(T+Te) of the demodulator 13 and the outputvoltage K cos φ(T) of the storage circuit 18 are simultaneously suppliedto the inputs of the respective comparators 23, 24. Each of thesecomparators 23, 24 has an input which is connected to ground and thesecomparators are used for determining whether the sine and cosine signalspresent at the other inputs are positive relative to ground. If the sinesignal at input 23 is positive, the output of the comparator selects thesine signal directly by means of switch 27. If the comparator indicatesthat the cosine signal at the input 23 is negative the signaloriginating from an inverter 25 is selected by means of a switch 27. Ifthe cosine signal at the input of the comparator 24 is positive, saidsignal is selected in the same manner by switch 28; if, on the contrary,said cosine signal is negative, the signal is inverted by means of theinverter 26 before it is selected by switch 28.

The various quadrants in which the sine and cosine signals have positiveand negative values are shown in FIG. 2. If the shaft of the synchrorotates counter-clockwise, the direction of rotation of said shaft ispositive and the signal H.sup.._(B) (derived from H_(B)), whichindicates the rate at which the barometric height changes, is a positivesignal, which means that the aircraft ascends. If, on the contrary theshaft of the synchro rotates clock-wise, said direction of rotation isnegative and signal H.sup.._(B) is a negative signal, which means thatthe aircraft descends. The system offers a solution for all these casesby always indicating the correct sign of H.sup.._(B).

The output voltages K·sin φ(T+Te) and K·cos φ(T) selected by switches 27and 28, respectively, are input parameters in the respectivelogarithmetic amplifier circuits 29, 30. The output voltages supplied bythese circuits, which are equal to log [K·sin φ(T+Te)] and log [K·cosφ(T)] are supplied to the positive terminals of an adder 31 for formingthe sum:

    log [K·sin φ(T+Te)]+log [K·cos φ(T)].

The output voltages of the demodulators 13 and 14 are also squared insquaring circuits 32, 33 for obtaining the products K² sin² φ(t) and K²cos² φ(t), which are thereafter added in an adder circuit 34 for formingthe sum K² ·[sin φ(t)+cos² φ(t)]. The output voltage of the addercircuit 34 is supplied to a logarithmic circuit 35 whose output voltagelog [K² ·{sin² φ(t)+cos² φ(t)}] is supplied to the negative terminal ofthe adder circuit 31. Thus, the output voltage of this adder circuit 31is given by the formula:

    log [K·sin φ(T+Te)]+log [K cos φ(T)]-log [K.sup.2 {sin.sup.2 φ(t)+cos.sup.2 φ(t)}].

This output voltage is supplied to a circuit 36 which forms theanti-logarithm of its input voltage for obtaining the expression:##EQU3##

As mentioned previously this expression is identical to the expressionsin φ(T+Te) cos φ(T). Via switch 37 the output voltage 36 is stored instorage circuit 39. Closing the switch 37 is effected during the shortperiod of time (some microseconds) in which the quotient is available atthe output of the circuit.

The output voltages of the demodulators 13 and 14 are sampled for thesecond time at approximately the instant (T+Te). As describedpreviously, sampling the voltage K·sin φ for the second time isperformed at the instant (T+Te) for computing the expression:

    sin φ(T+Te) cos φ(T).

As soon as the value corresponding to this expression is stored in astorage circuit 39 the switches 20 and 22 are simultaneously opened andthe switches 19 and 21 simultaneously closed, in order to supply theoutput voltages of the storage circuit 17 and of the demodulator 14 tothe respective comparators 23, 24.

The voltage K·cos φ is sampled for the second time at the instant(T+Te)+T₁, where T₁ is the instant at which the expression cos φ(T+Te)sin φ(T) is formed, T₁ being some microseconds. As stated previously,the fact that the value T₁ =(T+T₁)-T is small relative to Te=(T+Te)-T,the error this produces in the calculation can be neglected, so that itis assumed that the second sampling procedures are performed at theinstant (T+Te).

The comparators 23 and 24 examine the signals sin φ(T) and cos φ(T+Te)for selecting the positive values by means of switches 27 and 28. As inthe preceding case, the output voltage of the logarithmic circuits 29and 30 are supplied to the positive terminals of the adder device 31 andthe output voltages of the demodulators 13 and 14 are supplied to thenegative terminal of the adder device 31 via the squaring circuits 32and 33, the adder circuit 34 and the logarithmic circuit 35.

So the output of the adder device 31 supplies the expression:

    log [K sin φ(T)]+log [K cos φ(T+Te)]-log [K.sup.2 {sin.sup.2 φ(t)+cos.sup.2 φ(t)}].

Switch 36 supplies the anti-logarithm of this expression: namely##EQU4##

This second quotient is stored in circuit 40 by means of switch 39 whichis closed by a pulse during a time interval of a few microseconds. Thequotient thus stored in the circuit 40 is thereafter subtracted from thequotient 1 stored in circuit 39, this subtraction being performed in asubtraction circuit 41 to obtain the expression:

    sin (Δφ)-sin φ(T+Te) cos φ(T)-sin φ(T) cos φ(T+Te).

For small values of Δφ, sin Δφ is approximately equal to Δφ, this valuebeing the angle which is proportional to the speed of rotation of thesynchro. The embodiment shown in FIG. 1 is used in a digital systemwhich furnishes a warning relative to the distance to the ground andwhich comprises an analog-to-digital converter having a resolution of 11significant binary digits plus the sign, that is to say the maximumresolution is equal to φ=1/2047.

An aircraft in which the synchro is used for computing the speed atwhich the barometric height H.sup.._(B) changes, is usually providedwith systems which, in aeronautics, are usually called "KIFIS" of "FINESYNCHRO," 18000 feet altitude corresponding to one revolution of theshaft of the synchro in the "KIFIS" system, and 5000 feet correspondingto 1 revolution of the shaft of the synchro in the "FINE SYNCHRO"system.

So, for the system "FINE SYNCHRO" the angle φ corresponds to the changein the barometric height H.sup.._(B) : ##EQU5##

For the "KIFIS" system the corresponding change in H.sup.._(B) is:##EQU6##

Selecting the scale for the "FINE SYNCHRO" system or for the "KIFIS"system is performed by component 42 which is a scale-selection circuithaving an amplifier 43 and calibration resistor 46, 47. Choosing betweenthe "KIFIS" and "FINE SYNCHRO" systems is performed by a selectionsignal KIF/FS and is effected in the device which furnishes a warningrelative to the distance to the ground, in accordance with thespecifications of the ARINC 594 standard. When the "FINE SYNCHRO" systemis used, the scale factor is determined by the inverter 48 and theresistor 46 which are put into operation by closing switch 44. When the"KIFIS" system is used, resistor 37 is directly made operative by theclosing of switch 45 for obtaining the suitable scale factor. The outputvoltage of the scale selection circuit 42 is H.sup.._(B) and canimmediately be used for the device which supplies an indication relativeto the distance to the ground, after an analog-to-digital conversion.When the "KIFIS" system is used a 26 Volts, 400 Hz (VKIF) signal isadded, after amplification, in circuit 51 to the selection signal KIF/FSin an AND-gate 50 to produce a validity signal "KIFIS." If the "FINESYNCHRO" system is used no validity signal is usually produced; however,the signal at the output of the adder circuit 34 is combined in anOR-gate 23 and in an AND-gate 49 with the selection signal FS of the"FINE SYNCHRO" system for obtaining a validity signal "FINE SYNCHRO" atthe output. Certain barometric altimeters of the "FINE SYNCHRO" typegenerate, however, a direct current validity signal "FINE SYNCHRO"(VFS). This signal VFS is applied to the OR-gate 53 simultaneously withthe signal deriving from the adder circuit 34 for supplying a validitysignal "FINE SYNCHRO" to the AND-gate 49. At the output the two validitysignals are transmitted via an OR-gate 52 in the form of a validitysignal for the system "KIFIS" (SVKIF) or a validity signal for thesystem "FINE SYNCHRO" (SVFS) and are directly used for the device whichgives an indication relative to the distance to the ground.

The dashed lines show the various switches described with reference toFIG. 1, which are controlled and synchronized by the clock system 60shown in FIG. 1 by means of a rectangle.

FIG. 3 is a time diagram of the signals for controlling the switches andthe synchronization, the signals being supplied by clock system 60.

In FIG. 3 the time axis during the pulse duration of the pulses shown isexpressed in microseconds, and the time axis for the pulse intervals isexpressed in milliseconds as is shown in the diagrams CL1 to CL5.Diagram CL1 shows a clock signal generated by a monostable circuit whichgenerates a brief pulse. By closing the switches 15 and 16 (FIG. 1)during the pulse duration of pulse 61 the sine and cosine signals aresampled for the first time by the pulse 61, the leading edge of whichoccurs at instant T. Diagram CL2 shows that a pulse 62 is produced atthe instant (T+Te) occurring a time interval Δt after the end of pulse61. The dashed arrows in FIG. 3 show the time relation between thevarious clock signals. During the duration of pulse 62 the sine signalsare sampled for the second time at the instant (T+Te) by closing switch20 and the first expression sin φ(T+Te) cos φ(T) is formed by closingswitch 22 at last-mentioned instant. After a given period of time(approximately some microseconds) after the leading edge of pulse 62 andthe calculations of sin φ(T+Te) cos φ(T) has ended, a pulse 63 occurs asshown in diagram CL3. During the duration of pulse 63, which lasts for afew microseconds, the first expression is stored in a store 39 byclosing switch 37. Said pulse 63 is generated by a monostable circuit.The trailing edge of pulse 63 causes pulse 62 to end after a shortperiod of time, this pulse 62 starting the pulse 64 shown in diagramCL4. Switch 37 is opened as soon as pulse 63 terminates and switches 20and 21 are simultaneously opened at the termination of pulse 62. Theleading edge of pulse 64 coincides with the instant (T+Te+T₁) at whichinstant the cosine signal is sampled for the second time by the closingof switch 21. During the duration of pulse 64, which lasts for a fewmicroseconds, switch 19 is closed, simultaneously with switch 21, asdescribed for pulse 62, the second expression sin φ(T) cos φ(T+Te+T₁) iscomputed. This expression corresponds approximately with the expressionsin φ(T) cos φ(T+Te) as previously described. After a short delay theleading edge of pulse 64 produces pulse 65, which is shown in timediagram CL5, the second expression being computed during this timedelay. Pulse 65 which is comparable to pulse 63 is used for storing thesecond expression in storage circuit 40 by the closing of switch 38. Thetrailing edge of pulse 65 causes pulse 64 to be terminated. Switch 38 isopened as soon as pulse 65 terminates and switches 19 and 21 aresimultaneously opened as soon as pulse 64 terminates.

The pulse shown in the diagrams CL1, CL3 and CL5 can be derived by meansof controlled monostable circuits, while the pulses shown in thediagrams CL2 and CL4 can be obtained by means of bistable circuits.Pulse 61 may either function independently or coincide with the clockpulses of a device which gives an indication of the distance to theground. If, in the latter case, said warning device receives each second30 sampling values about the height H.sup.._(B), the period between theconsecutive pulses 61 is to approximately 33 microseconds. The period oftime T_(e) is approximately 15 milliseconds and the period of time T₁ afew microseconds. The output voltage of the signal H.sup.._(B) remainsconstant from the trailing edge of pulse 64 to the leading edge of pulse62 in the next cycle, which occupies approximately 33 milliseconds. Thevariations in the voltage H._(B) during the sampling procedure mayconsequently be neglected and the received values can be assimilatedfrom the instantaneous values of the samples.

If so desired, the period of time Te can be easily changed to obtained ahigher precision; this also applies to the pulse repetition period ofpulse 61 if higher sampling rates are required. The resolutions of 1.4foot per minute for the "KIFIS" system and 0.3887 foot per minute forthe "FINE SYNCHRO" system are no limitations particular to the describedembodiment with respect to the speed of rotation of the synchro, but arethe result of the analog-to-digital converter used in this embodiment;the use of analog-to-digital converters of a higher precision willincrease the resolution of the system, if so required. All switches areelectronic switches and, preferably, field effect transistors.

It should be noted that the described system only represents a specialembodiment of the invention. The sine and cosine signals which aresampled a first and a second time, said sampling operations beingperformed at the instants T, (T+T₁), (T+Te) and (T+Te+T₁) in whichTe>>T₁ could be directly applied to the input of a device, whichfurnishes an indication of the distance to the ground, via ananalog-to-digital converter operating in synchronism with the samplingperiod. A micro-processor implemented device for indicating the distanceto the ground could perform all computations required in accordance witha given program in the same order as described for the embodiment shownin the drawing for producing the required signal H.sup.._(B).

Inversely, it would be possible to implement a digital embodiment usingthe same principles as those described for the system shown in FIG. 1.In such an embodiment the sine and cosine signals can be sampled asdescribed with reference to FIG. 1, while all arithmetic units can bereplaced by an arithmetic logic unit in which a micro-programmable logicnetwork is used for performing the multiplications, divisions, additionsand subtractions required in the sequence already described. The storagecircuits can be replaced by registers. A further condition is theseparate, non-recurrent computation and storage of the expression:

    K.sup.2 [sin.sup.2 φ(t)+cos.sup.2 φ(t)]

in order to use this expression with each expanded product. This outputvoltage H.sup.._(B) can be directly sampled by the device which gives anindication about the distance to the ground.

What is claimed is:
 1. A system for generating a signal which is representative of the speed of rotation of a synchro using a first digital signal which is proportional to the sine of the angle of rotation of the shaft of the synchro and a second digital signal which is proportional to the cosine of said angle, comprising:means for a first sampling of said sine signal; means for a first sampling of said cosine signal; means for a second sampling of said sine signal after a predetermined period of time has elapsed after said first sampling of the sine signal; means for a second sampling of said cosine signal after a predetermined period of time has elapsed after said first sampling operation of the cosine signal; means for multiplying said second sampled sine signal by said first sampled cosine signal for forming a first product; means for multiplying said first sample sine signal by said second sample cosine signal for forming a second product; means for squaring said sine and cosine signals; means for adding said squared sine and cosine signals for forming a first sum; means for dividing said first product by said first sum for forming first quotient; means for dividing said second product by said first sum for forming a second quotient; and means for subtracting said second quotient from said first quotient for producing a signal which is representative of the speed of rotation of the said synchro.
 2. A system as claimed in claim 1, further comprising:means for electrically insulating said sine and cosine signals relative to the output voltage of said synchro.
 3. A system as claimed in claim 2, further comprising:means for demodulating said sine and cosine signals, said means comprising: a modulated reference voltage; first and second switching means for sampling said sine and cosine signals for the first time; first and second storage elements for storing said first sampled sine and cosine signals; third and fourth switching means for sampling said sine and cosine signals for a second time after a predetermined time interval following said first sampling operations; a multiplying means; a fifth switching means for simultaneously applying said first sampled and second sampled cosine and sine signals to said multiplying means for forming a first product; means for squaring said first sampled and second sampled sine and cosine signals; means for adding said squared signals for forming a first sum; a dividing means; means for applying said first product and said first sum to dividing means for obtaining a first quotient; a sixth switching means for sampling said first quotient; a third storage element for storing said sampled first quotient; a seventh switching means for simultaneously applying said first sampled sine signals and said second sampled cosine signals to said multiplying means for forming a second product; means for applying said second product and said first sum to said devising means for forming a second quotient; an eighth switching means for sampling said second quotient; a fourth storage element for storing said second sampled quotient; and, subtracting means for subtracting said second quotient from said first quotient for forming a signal which is representative of the speed of rotation of the synchro.
 4. A system as claimed in claim 3, further comprising:ninth and tenth switching means for selecting a scale for said signal which is representative of the speed of rotation of the synchro.
 5. A system as claimed in claim 4, further comprising:comparators for determining the polarity of said first sampled and second sampled sine and cosine signals.
 6. A system as claimed in claim 5, further comprising:eleventh and twelfth switching means for selecting the positive values of said first sampled and second sampled sine and cosine signals.
 7. A system as claimed in claim 5, further comprising:a clock pulse system for generating clock pulses for controlling and synchronizing said switching means.
 8. A system as claimed in claim 7, further comprising:means for generating a first pulse signal of a given time duration for controlling said first and second switching means; means for generating a second pulse signal after a given time interval has elapsed after said first clock signal for controlling said third and fifth switching means; means for generating a third pulse signal of a given time duration after elapsing of a time interval after the leading edge of the second pulse signal for controlling said sixth switching means; means for generating a fourth pulse signal after said second pulse signal has ended for controlling said fourth and seventh switching means; and means for generating a fifth pulse signal after elapsing of a given time interval after said fourth pulse signal for controlling said eighth switching means.
 9. A system as claimed in claim 2, wherein:said signal which is representative of the speed of rotation of the synchro represents the speed in which the barometric height H_(B) varies, proportional to the angle of rotation of the shaft of the synchro. 